This invention relates to an amplifier arrangement for amplifying an input signal, the arrangement comprising a first transistor having a control electrode coupled to an input terminal for receiving the input signal, having a first main electrode coupled to a first supply-voltage terminal, and having a second main electrode, and a second transistor having a control electrode, having a first main electrode coupled to the second main electrode of the first transistor, and having a second main electrode coupled both to a second supply-voltage terminal via a first current source and to an output terminal for supplying an output signal.
In general, such an amplifier arrangement can be used for amplifying a voltage in integrated semiconductor circuits.
Such a cascode amplifier arrangement is known, inter alia, from the book "CMOS analog circuit design" by P. E. Allen and D. R. Holberg, published in 1987. FIG. 6.3-1 on page 288 of this book shows a cascode amplifier arrangement in which the control electrode of the second transistor is coupled to a reference voltage terminal in order to receive a reference voltage. In the known cascode amplifier arrangement the first transistor converts the input voltage, which is applied via the input terminal, into a current which is proportional to the input voltage and which produces potential variations on the first main electrode of the second transistor. These variations, amplified by the gain factor of the second transistor, appear on the output of the arrangement. The overall gain is therefore equal to the product of the gain factors of the first and the second transistor. Consequently, the second transistor makes the gain of the arrangement higher than that of an amplifier arrangement comprising one transistor. This also results in an increased output impedance. In order to achieve a large unity-gain-bandwidth by means of the prior art cascode amplifier arrangement, large bias currents and transistors having a very short channel length are needed. However, this results in the output impedance and hence the gain being reduced substantially. Consequently, the prior-art arrangement is not capable of providing both a large unity-gain-bandwidth and a high gain.